Before you can understand fit, you need to understand placement. You have the saddle too far forward over your horse's shoulders. You can get away with that if the saddle has a lot of "flare" to the front of the tree. Most newer saddles (made in the last 20 years) do not have a lot of flare. Because of that, you need to put the saddle back so that those front conchos are about 1-2" BEHIND the back EDGE of the horse's shoulder blade. This allows the horse to have freedom of movement in the shoulders and prevent saddle sores.
If your horse has hollows behind the withers and most saddles "fall down" in to those pockets, then you need to get a saddle pad that will fill in for those hollows. Pro Choice makes one with extra padding there, or a built up roper pad, or something like a Skito Correction pad
(used over a thin blanket or felt pad).
Okay, so once you have placement figured out, you can look at fit. First, the saddle needs to be the right length. Once you have it placed on your horse properly, look at where the back of the saddle sits. The skirts should not be over the hips, or close to the hips. The back concho should be BEFORE the front edge of the horse's flank. If you feel around on the top of your horse's ribs, it should be about 1-2" before the last edge of the ribs.
So the saddle tree should be 1-2" behind the shoulder and 1-2" before the edge of the last rib. It can be shorter than that, but it should be no longer.
Next, you need to look at how it lays at the front of the horse. The front edge of the saddle should follow the line of the horse's side/shoulder, and be the same angle. You should have 2-3 fingers of clearance above the withers as well. The front concho should be somewhere on the withers, about halfway down, depending on the size of the withers.
Next, you need to feel under the saddle. Cinch it up lightly (not pad) and run your hand from the very front to the very back, under the saddle, with your fingertips an inch or two down from the spine. You should feel even pressure front to back, no tight spots or air pockets.
Finally, step back and look at the balance of the saddle. If you have a horse with hollows behind his withers, put your pad to fix it under the saddle, or fold a towel over and put it under the front. Look at how the saddle sits. The seat should be even and the pommel and cantle should be level.
Most modern stock horses need QH or Full QH bar saddles. Very few narrow stock horses or appendix types will need Semi-QH bars. For your horse, I agree, try both regular QH bars or Semi-QH and see how it goes. Each manufacturer and brand is different, and even some old saddles of the same brand will fit differently than newer ones. You're just going to have to try a bunch on!